For many viruses, the greatest anti-viral immunity arises from natural infection, and this immunity has best been mimicked by live attenuated virus vaccines. However, in the case of HIV, such live attenuated organisms may be considered too risky for uninfected human recipients because such retroviruses have the potential risks of integrating viral genome into the host cellular chromosomes and of inducing immune disorders. To reduce these risks, an alternative is to use pure, well-characterized proteins or synthetic peptides that contain immunodominant determinants for both humoral and cellular immunity. An important component of cellular immunity consists of class I MHC restriction CD8.sup.+ cytotoxic T lymphocytes (CTL) that kill virus infected cells and are thought to be major effectors for preventing viral infection.
Cellular immunity is also a key component of the mechanism of tumor rejection. No previous cancer vaccine has shown much success in treating cancer. Most previous cancer vaccines that have been tried have involved whole cancer cells or cell extracts, which are poorly defined mixtures of many proteins. Prior methods to induce CD8.sup.+ CTL with synthetic peptides have been limited to antigens from foreign microbial pathogens, such as viruses and bacteria.
Present theories of tumor initiation and progression hold that tumor cells arise from mutational events, either inherited or somatic, that occur in a normal cell. These events lead to escape from normal control of proliferation in the cell population which contains the tumorigenic mutation(s). In many instances, mutations resulting in substitution of a single amino acid are sufficient to convert a normal cellular protein into an oncogenic gene product. The normal genes which encode the proteins susceptible to such oncogenic mutation are called "protooncogenes".
Ras is a typical protooncogene. The normal protein product of the ras gene is a GTPase enzyme which is part of the pathway that transduces biochemical signals from cell surface receptors to the nucleus of the cell. Mutations which inhibit or abolish the GTPase activity of ras are oncogenic. For example, the Ala.sup.59, Gly.sup.60 and Gln.sup.61 residue of the ras protooncogene are frequently mutated in human tumors (80).
Previous methods for producing CD8.sup.+ CTL have not shown the feasibility of inducing CTL against proteins that differ from the normal, "self" proteins by only a single amino acid substitution. However, it is clear from studies of tumor-infiltrating lymphocytes in humans, as well as from animal model studies, that CD8.sup.+ CTL can eradicate cancers in vivo.
No previous studies have shown the ability to immunize with a mutant synthetic peptide from a natural endogenous cellular protooncogene product to induce CD8.sup.+ cytotoxic T lymphocytes (CTL) that can kill tumor cells expressing a mutant endogenous gene product. Several studies have shown the ability to immunize mice with peptides to induce virus-specific or bacterial-specific CTL (P. Aichele et al (69); M. Schulz et al (42); W. Kast et al (41); J. Harty and M. J. Bevan, J. (77); M. K. Hart et al (79), but with the exception of Harty and Bevan, these have all required the use of adjuvants and high doses of peptide. Furthermore, since viral or bacterial proteins are foreign to the host, and it is known that it is possible to raise CTL to these, it was expected that any viral peptide immunization that succeeded would result in CTL that could kill cells expressing the foreign viral protein.
However, for oncogene products, or products of mutated tumor suppressor genes, for example p53, which reside primarily in the nucleus, it was not clear whether the mutant protein would be produced in sufficient amounts in tumor cells. Nor was it known if the protein would be processed through the appropriate cytoplasmic pathway to be presented by class I MHC molecules to CTL. It had also been questioned whether a single point mutation in a normal, endogenous protein would be sufficient to produce a CTL response.